In systems wherein a loop circuit includes a number of loading elements, it is desirable to ascertain the number of loading elements which are in an open circuit condition. For this reason, indicating devices are employed to determine and display the number of open circuit loading elements. One such example of loading elements is a current transformer in which the secondary circuit of each current transformer is loaded with an electric incandescent lamp. Preferably, these lamps may serve in airfield lighting plants to illuminate runways, approaches etc.
Other examples of plants in which indicating devices may advantageously be employed include road lighting plants and plants serving to prevent railroad switches from being blocked up by snow by means of melting the snow. In the latter case, a number of rail switches are each equipped with electric heating units, each coupled to a secondary circuit of a current transformer.
With respect to airfield lighting plants, if m of the n lamps at a given moment are not alight this indicates that there must be a break somewhere in the secondary circuit of the respective transformer or transformers, i.e., these transformers are unloaded, and in most cases because of the lamp or lamps being burned out.
In preferred airfield lighting plants, the lamps are interconnected in series, but by means of individual current transformers. Notwithstanding the series connection, only the deficient lamps will not be alight.
Nevertheless, it is undesirable when in an airfield lighting plant there are lamps which are not alight. It is evident that it is not as undesirable if only a low percentage of the lamps fail. But if, e.g., 10% of the lamps fail it may be troublesome, and if the percentage is as high as, e.g., 20% it may be disastrous. In that instance, the pilots may have difficulty in getting an idea of the lie of the land, particularly during landing operations under low visibility conditions.
It has therefore been found desirable to maintain vigilance on the number of deficient lamps in such a plant by means of a suitable indicating device serving that purpose. Such device should preferably be designed for remote monitoring of the operational state of the lamps and might advantageously be installed with the maintenance staff of the airfield who would have to replace burned-out lamps in due time.
Such an indicating device has in fact been developed. One such device is disclosed in British Pat. No. 1,379,077. Therein the known device is operable in dependence upon the phase difference between the loop current and the loop supply voltage. However, the output signal produced in the known device is only a true indication of the number m of unloaded current transformers as long as the curve form of the loop supply voltage u does not change and as long as the loop current i is kept constant.
These facts render the known indicating device less applicable for use in connection with plants equipped with an alternating current source of a type yielding a current having a curve form differing from that for which the indicating device is designed, and in connection with plants equipped with an alternating current source having known first means for automatically keeping the yielded current constant at a fixed value irrespective of the number of current transformers being unloaded and having known second means for manually changing the fixed value for the constant loop current. Such change is required when the weather conditions, with respect to visibility changes, makes it desirable to change the luminous intensity of the lamps accordingly by manually increasing or decreasing the fixed constant value, as the case may be.
In the known indicating device a change in curve form and/or in the fixed value for the constant loop current will change the factor of proportionality expressing how the phase difference between the loop current and the loop supply voltage varies with changes in the number of current transformers being unloaded. Hence, the reduced reliability of the known indicating device is evident.
I have invented an improved indicating device and method of operating same which successfully avoids the above-mentioned disadvantages and limitations of known prior art devices. In particular, the indication device according to the present invention does not suffer from the inconveniences mentioned above and the output signal it produces is not determined by the phase difference between the loop current i and the loop supply voltage u.